Many optical systems utilize optical fibers to deliver light from a remote light source to a target destination. In a typical system, the light source is coupled to the fiber and light supplied by the source is guided by the fiber to the target destination. Optical fibers have been widely used in telecommunications to deliver information encoded in the form of an optical signal. A telecommunications link includes a transmitter that converts an electrical signal to an optical signal. The optical signal is launched into the fiber and transmitted to a receiver that reconverts the optical signal back to an electrical signal for further processing at the destination end of the link. Optical fibers have also been used as point illumination sources. In these applications, light from a source is coupled to the receiving end of the fiber and emerges from the destination end of the fiber as an illuminating beam.
There has recently been interest in extending the use of optical fibers to applications in broad-area illumination. In these systems, the objective is to achieve controlled release of light along at least portions of the length of the fiber. Instead of using the fiber to confine light and transmit it with minimal losses from a source to provide point illumination to a target positioned in the direction of the fiber axis, the objective is to use the lateral surface of the fiber as a broad-area source of illumination that operates in the radial direction of the fiber.
Light-diffusing fibers are a class of fibers that can be used as a broad-area illumination source. Light-diffusing fibers are designed to scatter light propagating along the fiber axis in the radial direction. Radial scattering is accomplished by incorporating nanostructural voids within or throughout the core and/or cladding regions of the fiber. The voids are low-index regions, typically filled with a gas, and have dimensions on the order of the wavelength of the light propagating through the fiber. The refractive index contrast between the voids and surround dense glass matrix effects scattering of the light. The scattering efficiency, and hence intensity of scattered light, can be controlled by controlling the dimensions, spatial arrangement and number density of voids. In addition to broad-area illumination, light-diffusing fibers can be employed in displays and as light sources in photochemical applications. Further information about light-diffusing fibers and representative applications can be found in U.S. Pat. No. 7,450,806 and U.S. Pat. Appl. Pub. No. 20110122646, the disclosures of which are hereby incorporated by reference herein.
Light-diffusing fibers are versatile and compact sources of broad-area radial illumination and offer the further advantage of maintaining functionality when bent. This allows light-diffusing fibers to be deployed as illumination sources in tight spaces and areas where it is impossible to deploy conventional light sources. In order to achieve functionality in restricted spaces and in bent configuration, the diameter of the fiber needs to be small. The central glass portion (core+cladding) of a fiber typically needs to be on the order of 125 μm or less to avoid fracture upon bending.
One of the drawbacks associated with the low diameters of light-diffusing fibers is poor coupling efficiency to LED (light-emitting diode) sources. With the increasing trend away from conventional incandescent light sources, LEDs are expected to become increasingly important. It would be desirable to have a diffusive lighting element capable of coupling efficiently to LED light sources.